arm/dt: msm8974: Add the vdd-cx property to tspp only on V2

MSM8974V2 hardware requires the CX rail to be elevated to a
higher voltage before the TSIF clocks are enabled.
Add the vdd_cx-supply property to the tspp node,
allowing the msm_tspp driver to set the voltage.

Change-Id: Ia05b8d5992122c742729039203c89617aeab5afa
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>